The invention relates to tungsten chemical mechanical polishing and in particular to tungsten CMP solutions having a controlled static etch rate. Tungsten CMP slurries rely upon both tungsten etch and mechanical abrasion to planarize the tungsten's surface. Competing chemical reactions take place during tungsten CMP. The first of these is an oxidation reaction. During oxidation, the oxidizing agent acts to form a tungsten oxide with the surface of the substrate. The second reaction is the complexing reaction. In this reaction, the complexing agent actively dissolves the oxide film growing on the substrate from the oxidation reaction.
Because of tungsten's high stability, tungsten slurries must typically rely upon powerful oxidizers. In view of this, strong oxidizers such as halogen oxides have been used or proposed as oxidation agents for tungsten polishing slurries. For example, Streinz et al., in U.S. Pat. No. 5,993,686, disclose oxidizing metal salts, oxidizing metal complexes, nonmetallic oxidizing acids such as peracetic and periodic acids, iron salts such as nitrates, sulfates, EDTA, citrates, potassium ferricyanide, hydrogen peroxide, potassium dichromate, potassium iodate, potassium bromate, vanadium trioxide and the like, aluminum salts, sodium salts, potassium salts, ammonium salts, quaternary ammonium salts, phosphonium salts, or other cationic salts of peroxides, chlorates, perchlorates, nitrates, permanganates, persulfates and mixtures thereof. Similarly, Mravic et al., in WO 99/67056, disclose the use of hydrogen peroxide, potassium ferricyanide, potassium dichromate, potassium iodate, potassium bromate, vanadium trioxide, hypochlorous acid, sodium hypochlorite, potassium hypochlorite, calcium hypochlorite, magnesium hypochlorite, ferric nitrate and mixtures thereof. These oxidizers such as, halo-oxides chemically react with the substrate surface to form a metal oxide. Then slurry abrasive from the CMP process removes the inhibited tungsten oxide from the surface of the substrate. In this manner, the CMP process removes material from the substrate and planarizes its surface.
Iodate-containing slurries used in tungsten CMP have the ability to inhibit the static etching process. Unfortunately, although iodate-based slurries succeed in inhibiting static etching, they also have the following undesirable properties: 1) requiring aggressive alumina particles to remove the tungsten oxide that can result in scratching; 2) requiring appropriate equipment for iodine removal in accordance with established environmental regulations; and 3) contact of the slurry with polishing equipment results in the equipment turning an undesirable brownish-yellow color.
In U.S. Pat. No. 5,958,288, Mueller et al. conclude that iron-containing oxidizers, such as ferric nitrate, can act as a catalyst to promote tungsten removal when iron is present in an amount less than 3,000 ppm. The problem with these slurries is that tungsten static etching is a common side effect. After the CMP process, the metal polishing slurry that remains on the surface of the substrate continues to etch the substrate. Sometimes, static etch has beneficial consequences for some semiconductor integration schemes. But in most instances, minimizing static etch improves semiconductor performance. In addition, static etch may also contribute to surface defects such as pitting and key-holing. These surface defects significantly affect the final properties of the semiconductor device and hamper its usefulness.
Grumbine et al., in U.S. Pat. No. 6,083,419, disclose the use of nitrogen containing oxidizer to control static etch. Unfortunately, as far as known, these compounds have only a limited impact upon static etch rates. At present there is an ongoing need for a tungsten polishing slurry that has both a rapid tungsten removal rate and limited static etch. In addition, there is a need for a polishing slurry that also eliminates the scratching, environmental and cosmetic issues associated with iodate-containing slurries.